In the packaging industry there is a high level of demand for transparent biaxially oriented polyester film. If polyester film is used to pack foods which are fresh and are highly perishable a possible consequence especially in the case of refrigerated goods is undesired condensation on the film and resultant impairment of transparency, when moisture from the packaged product condenses in the form of droplets, usually of various sizes, on the film. The condensate here markedly impairs the transparency of the film. A hydrophilic coating, also termed an antifog coating, can be used to inhibit transparency reduction on the film.
Various technologies can be used to achieve antifog properties on plastics surfaces. A first idea uses addition of an additive to the polymer, and secondly an antifog coating can be applied to the film. Addition of an additive to the polymer is possible in articles comprised of polyolefin, in particular films (e.g. WO 2002/074535). The antifog action on the film surface is based on the migration of the additive, usually an amphiphilic molecule, to the surface, the result being that the polar ends of the molecules form the actual antifog layer. This principle cannot be applied to a polyester film because the polyester is highly polar.
EP 322 529 describes a copolyester coating inter alia for polyesters, comprising up to 95% of isophthalic acid and from 5 to 35% of a monomer having an alkali metal sulfonate group. The coating is used as adhesion promoter with respect to vapor-deposited metals. Nothing is said about surface tension of the coated film or about behavior with respect to water condensation.
EP 1 358 896 describes a coating for a needle which comprises a sulfonated polyester and comprises a surfactant. In the examples, these two components are mixed in a ratio of about 10:1 and applied by means of a dipping process to the substrate. The contact angles of the coated substrates with respect to water are smaller than 10°, i.e. surface tension is greater than 70 mN/m. Nothing is said about the thickness of the coating.
DE-A-10 2004 049 609 describes a coating which comprises polyvinylpyrrolidone, a surfactant, and optionally an adhesion-promoting polymer. This composition is suitable for the in-line coating of biaxially oriented polyester films. The resultant coated films feature a characteristic hydrophilic surface which inhibits condensation of water droplets on the film. The optical quality of the coated film requires improvement. The coating increases the haze of the film, and this is undesirable particularly in the case of high-transparency films.